Means and method for maintaining constant transmission efficiency



Jam.v 14, 1930. l

M. K. -Z'INN MEANSAND METHOD FCR MAINTAINING CONSTANT TRANSMISSION EFFICIENCY 5- 2M/Wonen [any INVENTOR ff/#Ma ATTORNEY Sheets-Shed@ 2 Patentecliden.v 1,4, l30

UNiTEo-siATss- PATENT' ourlisci;vv y

iviANvEL K. ZINN, ornnooKLYN, NEW YORK, AssieNon To AMERioAN'TnLEPHoNs AND TELEGRABH oo MPANY, A conronATioN or Nnw Yomrq" M EANs AND METHoD non MAINTAINING CONSTANT TRANSMISSION iirforENoY Application led October 20, 1928. Serial o. 313,870. p'

This -invention'relates to maintaining constant transmission eiiciency at relatively high frequencies on transmission circuits, and is more particularly applicable to high grade i program transmission circuits` which are 'loaded to permit of the transmission of'a wide y 1 band of frequencies.

The development of radiobroadcastinghas made 'necessary the transmission of wider 10 ranges of frequencies over certain types of telephone circuits in order thatmusical programs may'be transmitted without distortion.

In certain casesit is necessary to-load.these program circuits in order'that a suhciently i wide band'of frequencies may be transmitted withoutffundue attenuation.v To maintain aA uniform'transmission loss on circuits of this type, atfal-l frequencies, under different temperature conditions, requires amore elaborate 2o method of. transmission regulation than the so-called pilot'wire-method which is commonly employed for ordinary ,telephone cirr cuits. The pilot wire method of regulation in-` volves the provision of a circuit having characteristics similar to the circuits to be conf trolled and subected to the same changes in temperature.Y s the temperature changes,

the resistance-.ofthe pilot wire changesy and 3o automatically'l controls the adjustments of potentionieters associated with regulating repeaters in the 4controlled circuits to compen. sate. for the change inv transmission eiiiciency, due to the change in temperature. For a given change in temperature, the change in transmission will be different for different frequencies. The pilot wire regulating system is based upon the priniple that if'the direct current resistance of thev circuit is known, its temperature is known; and `once they temperature is known, the transmission loss at all frequencies is known. When this proposition holds true it is possible tol design the steps on the potentiometer to 'give proper increments of, gain so 'that the net loss. of the circuit may be maintained at aconstant' value for allfrequencies at any temperature. This is accomplished by making the, potentiometer steps not ure resistance (which would introduce a ciange in the repeater gain which would be constant for allfrequencies) but a combination of-'resistance and reactiveele-A ments. y 1 n For ordinary telephone transmission, such a method of regulation hasbee'n-found to be adequate. At higher frequenc1es, however,

' this method of regulation is inadequate-as it involves the assumption that the 'forniof the transmission-frequency curve-at anytemper-v ature is always the same. The behaviorof'eo' the loading coil resistance, however; intro'- L duces factors which make the vtransmission frequency characteristic dependl .upon the history7 o f the cable with respect to temperature'. That is to say, the-transmission frequency characteristic, particularlyat, the higher frequencies, may not algwa s be' thev same at a given temperatura'dependin u on whether the temperature is-risingorallling at the time the given te'irip'erature 'is reached. If the pilot wire circuit 1s adjusted to give a fiat transmission characteristic at a given temperature7 -it may `not be,iiat when 'that temperature is-'reached again under a diiferent set of preceding conditions;

The principal factor responsible for these diliiculties is the `fact that, when the'teni-v perature is changing, the temperature of the loading coils tends to lag behind that of the cable by reason of the heat-insulatin property of the compound with which the 2loading coil cases are filled. The result is-.that the loss at high frequency may increase when the temperature increases and decrease when the temperature decreases, .even though the net loss characteristic has been adjusted to be `flat on the assumption of the same temperature f or'jboth yCoils and cable.

It is therefore proposed to supplement the pilot wire regulation apparatus by a high frequency-'j` pilot channel regulator designed to introduce into thel repeater gain characteristic the proper amount of twist to compensate 'for the distortionat high frequencydue to the lag in temperature of the loading coils with respect to the cable itself. This 'pilot channel arrangement involves the transmission of apilot frequency above the signal range to be transmitted (but below the cut-off of the loading system), and the 10U' pilot frequency controls mechanism to prof i vide a continuous automatic means of ad-l justing. the high frequency vgaini'of the regf ulating repeater in sucha Way as to maintain a constant value of net loss at the pilot frequency. Vhen the pilot Wire regulator operates in response to a change in temperature, and adjusts the potentiometer to restore the transmission frequency characteristie to its normal at-condition, any de parture. of the circuit from this flat condil tion, dueto a lag in the temperature of the loading coils, Will result in'a .change in the 'normal transmission level of the pilot channel. Consequentlythe supplementary pilot 4channel 'regulating apparatuswill be ad- )listed `to introduce suitable networks. to

bring the pilot frequency back tol normal.

The inventionmay now be :more fully un derstood from 'they follovving'l descriptionwhen vreadin connectiony with the accom-k panying drawing in which Figure l is a circuit arrangement embodymg one form of Y the' invention; Fig. 2 is a series of curves illustrating the principles of the invention.; Fig. 3 isa series of curves 'showingthe component elements of the resistance of a loading coil; and Fig. 4 `shows a series of curves illustrating the character of theadjustments made'by the pilotxfrequency apparatus.

For good program transmission it may be desirable tol transmit a range yof frequencies to about 8,0Q0cycles per secr extending u l ond. In or el" that 'the loaded circuit may 'properly transmit such a band, the loading vsystem'shouldh be so designed as to have a -cut-off at about-115,000 cycles.A The attenl v-u'ation loss of such a loaded line, at the more important frequencies in the middle of the transmitted range, is approximately pro portional t'o the total 'resistance of the line including the resistance of the loading coils- At low frequencies the increment in the loss for a given resistance change isless than that at the middle frequencies, While at high frequencies it is greater. '1

The change'in attenuation for a given temperature chan ej is due to a change in the resistance of t e line wires, to a change in the line leakage, and to a'change inthe resistance of the loading coils. The change in line resistance with temperature is independent` of the frequency', and the same holds true -for the copper resistance of the' loading coils'. Referring to Fig. 2, the curve a represents the attenuation change at various frei quencies'due to the copper resistance of the 4line and loading coils fora given change in temperature.' The change in attenuation due to line leakage for a given change in temperature, may be`as represented by curve l) of'Fig.- 2.

' -The total .change in attenuation for a given temperature, also depends upon certain other characteristics of the loading coils Lacasse other than the copper resistance. If' a change in temperature produces a change in the line resistance AR, a change in the line leakage AG and a changeV in the loading coil resistance RC, then' the change of attenua- AGZ, 2

.at frequencies between .3 and .8`of the cutf. .off frequency. In the foregoing equation, AAa is the change `in attenuation, Zo the imrpedance of the' loaded line, andS the spacing of the loading coils.

The chan-ge in the loading coil resistance IRC may be divided into three components,

vthe direct current or copper resistance of the coil, a resistance due to hysteresis, and a rethe full line curves shoW how these factors vary with frequency at a given temperature Tand the dotted line curves represent the same factors When the temperature is tion of theloaded line that results from the temperature change may be Written approx'-` sistance due to eddy currents. In Fig. 3,

changed to T, the temperature T being higher than T. The direct current or` copper resistance is constant at all frequencies and 4has a value represented bythe curve It .of F ig.' 3 at a given temperature T. When i perature is increased to T', the resistance com' ponent due to hysteresis will be. as represented by 7c. From this it will be seen that the percentage change in the resistance component due to hysteresis, for a given changein temperature, is the same at ally frequencies.

The resistancecomponentv of theloading 'coil due tothe eddy current loss A'at a given temperature T, increases With the square of the frequency as represented by the curve e. lVhen the temperature isincreased to T', the resistance componentdue to .eddy current losses .will vary withfrequency, as indicated bythe curve e. A comparison of the ycurves c and e shows that the resistance component due to eddy currents decreases with anincrease in temperature although it increases With frequency.

The total effective resistance of ingl coil at a given temperature T, due to th'e several factors above enumerated, is represented by the curve-'ml of Fig. 3, andthe cor` the load- I f 4.1135260, I f. f

responding total resistance 'ati-the vtemperature T is represented by the dotted line curve decreases .with an increase in temperature over the higher frequency range, as shown by curves m and m of Fig. 3. i

In considering the changes inaattenuation of the loaded line as a whole,.it' is simpler to treat -the attenuation due to the directcur-` rent resistance of the loading coil-(ewhich"is approximately constant with frequency) Tas apart of the copper resistance of the line as a whole. As has alreadyl been stated, the attenuation chan e, due to the copper resistance ofthe line an coils for a given. change in temperature from, say, T to T', will be as .represented by the curve a of Fig. 2. The corresponding attenuation component due to line leakage will be represented by curve b. The attenuation change due to the hysteresis and eddy current components of theloading coils for a given change in temperature from T to T', will then be as, represented by the curve c 4of Fig. 2. It will be noted from the curve c that this change in the attenuation constitutes an increasel in attenuation for frequencies up to about 2,000 cycles, but for frequencies higher than 2,000 cycles, actually constitutesa'decrease in attenuation. This, of course, is tQ be expected from a consideration of curves m andm of Fig. 3.

If, now, we combine all of the factors entering into the lchange in the attenuation due to a given change in temperature, it Will be found that the increase in attenuation varies withfrequency as' represented by the curve d, which is the summation of the values represented by the curves a, b, and c. It will be understood, of course, that the curve d represents the vchange in attenuation for a' given change in temperature and that' for different temperature 'changes there will bea family of curves-similar-to the curve d, one corresponding to each change in temperature. For each-:of these curves the attenuation varies with frequency. Obviously, if the shape ofthe curve corresponding to each given teinperaturewerefixed, it would be possible to keep the attenuation constant at all frequencies, regardless of change in temperature, by means of the pilot wire method. This method depends upon adjusting a potentiometer network in accordance with'changes in the resistance of the pilot wire, the theory being that when the resistance is known the temperature is known, and when the temperature is known the curve corresponding to d is known. Therefore, all that is necessary is to include in the circuit some'element. which will produce a complementary change in at. tenuatlon such as 1s represented by the curve e of Fig. l1, thus bringing the over-all attenuation. change to zero, 'as represented by the curve'g. I

The pilot wire arrangement-for -accomf plishing this result i. is illustrated 'jin'l' Fig.v

1. 'In this jfigure, .L2 vfis asection ,of 75 line to be regulated, `thesaid linefinter; connecting lin-e sectionssuch' and L3. A repeater Rfds interposed'fbetween the line sections Lg. and L1, and regulating re` peaters comprising amplifiers R2 a-IidR2 are so .interposed etwcen the dine sections L2 and-` L. The. pilot wirecmprises a linesecti'on Lp which will be included in thesamecable' and will be loaded andotherwise constructed in the same manneras the line section L2.

The pilotl wire L, comprises with 'the resistance r4 one of the arms of a Wheatstone bridge of which r1, r2, and r3. comprise. the other three arms.v A volt-meter relay VRpis bridged across two op aosite terminals l and 3' 90 of the bridge and a'- attery B is `connected from the terminal 2 to an adjustable termi# nal 4 which may be shifted to balance'the bridge. The-voltmeter relay VRD controls the circuits of a motor Mp which drives a shai't 95! to shift the movable contact 4 along the resistance 7'., of the. bridge. Consequently, when the resistance of the pilot wire' Lp changes with temperature and the bridge becomes unbalanced, the voltrneter VRD closes one or the other of its contacts', depending upon the vsense in which the bridge is unbalanced, to

drive the motor forward or backward, as the case may be, until the bridge is automatically 'The operation of the motor Mp also drives the switch-arm 5 overa series of'contacts which are connected to various -points on the potentiometer l?. As the switch-arm 5 is connected with the -grid lof the repeater R2, 11

the shifting of the switch 5 along the contacts leading to the various points of potentiometer P changes the gain of the repeater according' ly. On the assumption that the resistance of the pilot wire changes with temperature' in a 115 similar manner to that of the line'section L2, the result of this action is that the changein attenuation due to the change in resistancepf the line section L2, will be compensated for by If the potentiometer P consisted of pure resistance, the change in gain ofthe amplifier would be uniformwith frequency, whereas it should vary with frequency' in such a manner y l) With such an Yarrangeme ntz the Compensap 2;.;1

balanced by the adjustment of the vcontact 4.

the adj ustmentof the gain ofthe amplifier R2. ma

lion for each change in temperature could be taken care of provided that the curves, such as l d, would always be the same shape when the circuit arrived at that temperature. For crdinary telephone' purposes. where frequencies not exceeding about 3,000 cycles arc involved, it has been found that the curve (Z is nearly enough the same shape under all conditions, so that the regulation obtained by the pilot Wire scheme above described is sufficiently accurate for practical purposes. When, however, We are dealing with frequencies'` as high as 6,000 or 8,000 cycles7 the behavior of the loading` coil resistance introduces factors which make the variation loss characteristic, represented by the curve CZ, depend upon the liistoi-y. of the cable with respect to .temperature. The variationloss at these high frequencies may Anot always be the same at a given temperature. depending upon Whether the temperature is rising or falling at thetime the given temperature is reached. .f This is due principally to thev fact that, as the temperature of the system changes, the temperature of theloading coilstends to lag behind that of the cable conductQrs b reason of the heat-insulating property of tA e corri-F;

pound with which the loading coil cases are filled. Consequently, when We have a cha'e in temperature which y'vould produce attenuation changes corresponding to the curvesa andas a result of the change in copper resist ance and the change in line leakage, the com- -ponent of the attenuation change due to the eddy current and hysteresis effect of the loading coil, will not attain the value indicated by the curve c but Will attain the. lesser value indicated by the curve c', due to the fact that the loadingcoil has not been heated up to the same point as the line conductors. This `tends to produce `an increase in the attenua- 1tion loss at the higher frequencies, dueto anincrease 1n temperature as indicated by the curve d of Fig. 2. Therefore, when the pilot wire arrangement adjusts the potentiometer to produce a compensating effect in accordance With the curve e, the net attenuation4 change' instead of being zero, as representedl by the curve g, becomes that represented by` the curve g of F ig. 2. The curve g may be; higher or lower than indicated at the higher frequencies, depending on the amount that the loading coil temperature lags behind the temperatureof the line conductor.

The curvcsof Fig. 2 are drawn on theas- Y sumption that the temperature is increased to a given temperature. If, however, the temperature had been above this linal tempera- .ture and had decreased to the same temperature, a series of curves similar to that of Fig. 2 but having vopposite .signs would result, so that in this case the net attenuation change represented by the curve -g Would be' in the direction of a gain rather than a loss. The result is that With the pilot Wire arrangement the net attenuation change instead of being a straight line such as the curve g, may have a twist either up 0r down, depending on vWhether the temperature is rising or falling,'due to the lag inthe temperature of th loading coil. This is particularly noticeable at the higher frequencies, and consequently, it is not possible by means of the pilot Wire regulator 'to' produce asufficiently accurate compensation for thezchange in attenuation due to a change in temperature.

It is therefore proposed to supplement the action of the pilot regulator v by high frequency pilot regulation. For this purpose a high frequency pilot current is transmitted S0 over the circuit to be regulated, the pilot frequency being preferably .just Vabove the range of frequencies to be used during transmissi'on.

If, therefore, after the' pilot Wire arrangetemperature, the net attenuation change departs from z ero, as represented by thecurve g or any similar .curve (eitherl gain or loss) the amplitude of the high .frequency pilot current will be correspondingly .changed andv e0' vcan produce an auxiliary regulation as Will now be described.

`For this purpose the pilot current from vthe source O1 is applied to the line section L2 through the. secondary of' the transformer 95 T1. This current is then transmittedthro-ugh' the repeater R1 over 'the line L2, through first stage of the regulating repeater R2, over the potentiometer P, through the second stage of.

the regulating repeater R 2 to the primary of '100 i the transformer T2. 'A 10W pass filter LPF2 is interposed between the transformer T2 and the transformer T3 to prevent the 'pilot cur-=irent being transmitted to tlie line section La. The pilot currenttherefore is transmitted over conductors 10 through the .amplifier A2` and high pass filter .HPF2' toa detector D2" in Whose output circuit' is arranged one Winding of a polar relay PE2. A'pull Will therefore be producedbythe Winding of the po- 1.1.0

lar relay PR2 Which-is proportional tothe yamplitude of the high vfrequency pilot current transmitted over-the line section L2.

As the armature of the polar relay PR2 should be` in neutral osition for a certain normal amplitude of t e pilot current, aeurrent of fixed amplitude is supplied by the generator O3 through the ad or artificial line N 3`having the desired net oss at the pilot frequency for the circuitbetween a and a. This c'urrent is then transmitted from an amplifier A3 similar to the amplifier A2 through a filter. HP-F2 similar to theA filter HPF2 to the detec-A tor D3- which has vin its output circuit an-.`. other winding of the polar relay PE2. The current supplied by the source O3 may also be ltransmitted over the next line section L3 as the high 'frequency pilot current for that 4 sectiomby means of the connections shown.

With the arrangement above described, the

polar relay PR2 will -operate whenever thel attenuation of the circuit between a and a departs from the .normal value at the pilot frequency. lf the attenuation increases, the armature of the relay will be shifted in one direction and' if it decreases, it will be shifted in the opposite direction. When a Change in .temperature of the circuit occurs, the normal pilot wire regulator will operate to readjust the potentiometer P. If, after this readjustf ment, the attenuation of the Circuit is normal at the pilot frequency (as it would be if the resultant net attenuation change corresponded to the curve/g of Fig. 2), the polar relay PR2 would not kbe actuated. 1f, however, there 'isla twist up or down in the curve correspending tol g after the ,pilo't wire regulator' .is operated,.the pilot channel apparatus will cause the pola;` relay PREv to' shift its armature '1 'in one direction or the other, depending upon whether the twist is up or down. The polar Arelay PR2 controls a motor M whichdrives a switch arm 6`over-a series of contacts which bridge one or the other of several networks of 2a high frequency distortion corrector'HFD across the circuit between the and R2. l Y

repeaters R2 ,Y 'x When the armature oli' the polar relay-PR2 is in neutral position, the switch arm 6 rests f upon its zero contacts and none of the networks of the'distortion corrector HFDZ will be bridged across thecircuit. If, however,l

the amplitude of the pilot frequency departs from the normal as represented by the line g.

of Fig. 2, after the pilot wire' arrangement has been readjusted in response to a change inl feachtype may .be employed as `:conditions temperature, the polar relay PE2 will cause the motor M to shift the arm 6 to one of the contacts serving to bridge the network of the high frequency 'distortion corrector across thecircuit.

The high frequency distortion `corrector HFD, as shown, includes two eac of two dilerent types of networks, although it will be understood that anyparticular number of may require.' When the temperature is risi s :li-ng, and the'loading coils lag in temperature behind the rest of thecircuit, the net at-J tenuation curve of Fig; Qwill have a rising characteristic'so that the; loss increases with frequency. To odset this -the motor M cuts in o ne oi the networks 01 or m2,: which vwillhave a complementary fallingcharacteristic as indicated bythecurves of Fig; 4. 1f, on the other hand, the temperatureis-falling, the

net attenuation change curve corresponding to g of Fig. 2, will have aiallingv characteristic, iii-which case the motorM acts to cut in onel of the networks such 'as y1 or y2 whichhave a rising characteristic.

-' The operation ,of-'the pilot channel arrangement, including the polar relay PE2,

'motor M, and high'frequency distortion corrector 'HFD, is to correct a twist up'. or

down in the frequency transmission characteristic ofthesystem which results from any adjustmenty of the potentiometer P bythe pilot wire arrangement. The pilot wire ar-v rangement will" of course-be common to a number of circuits, so that by .providing the shaft of the motor M with a' number of switch arms corresponding to 5, the potentiometers of other line sections in the saine cable as `the line section L2 may be regulated. In a similar manner the pilot frequency apparatus associated with the l'ine section L2 may serve for agroup ofsimilar-lines by arranging the vmotor M to drive switches corresponding .t0 6

for such othe lines! This will result in making thesameadjustments of-the high frequ`en cy distortion correctors of'each of the lines-.

subject to the same conditions.

l A,line used for -broad'castin'gmusthandle a,wide.ran e: in electrical power. level, particularly Wren' orchestral music is being' transmitted.` For such service, a long loaded l linegmay introduce appreciable distortion beresistance,'and therefore, the attentuations are greater for strong currents thanji'or weak currents.- (The eddy'curr'ent loss does not` vary with current strength.) i Consequently,

a variation in the attenuation of-a loaded line.

results from change inthe power level, Vthis variation being of a similar type to that represented by the curve 'g' of Fig,` 2 although at any given instantA it may be ina direction@ to either oppose or laid the :distortion due to vthe lag in temperature ofthe' loadin'gcoils.

The pilot frequency arrangement may also i Y serve to correctv'for the distortion introduced-.

by the change in power level for the obvious reason that the pilot frequencyis responsive.. .to any departure of the net -attentuation` curve g from its normal zero position whether it be caused by variation' in lthe power level Vor 'j by lag in the temperature of the loading coil.

The high frequency pilot arrangement serves to correct the twist in the high frequency ment of the pilot wire;

frequency distortion corrector HFD are cle- 'signed to take care of the eliect in the change ofpower levelv as well as lag in the tempera.4 .ture of the loading coils,- to provide a separate pilot channelapparatus for each circuit to` be corrected. This is for the obvious reason that the changes inthe power level are not the same on all of the circuits in the same cable range regardless of the temperature adjust- It will, of course, be v necessary, where the networks of the 'high grams. It Will be obvious that the general prinsulting froina" change in temperature, an adjustable transmission element in said loaded line, means to adjust saldtransmission eleciples hereindisclosedmay be embodied in ment in'accordance With the indicated change many other organizations Widely different from those illustrated, Without ydeparting from the spirit of the inventionas defined in thefllovving claims. l

That is claimed is: l. Ina system for regulating thetians-v mission over a'loa'dec'l line in which in re-v sponse to a given change in temperature the resultant transmission change varies With frequency, lthe methodwhic'h vconsists in observing the change in resistance of a circuit similar to the loaded line. to be. regulated, adjusting. a variable transmission element of the loaded line, to produce a transmissionchange corresponding to the observed change in resistance, and varying With frequency in a manner complementary to that of the loaded line, on the assumption `that' the loading coils have undergone the same temperature change as the otherelements of the line, transmitting a pilot Wave over 'a circuit liaving'the same electrical characteristics as the loaded line 'to'be` regulated, observing 'the variation of the levl ofthe received pilot vyave from the level it should have under the assumed condition after theadjus'tme'nt indicated by the observed resistance change. has been made, and adjusting another adjustable transmission element of the loaded line to compensate for the observedchange in level.

2. In' a system for regulating the trans-4 mission over a `loaded line in Which' in rej sponse to a given change'in temperature the resultanttransmission changevaries With frequency, the method Whichv consists in observ-J ing .the change in rsistance ofl a circuit similarto the loaded line to. be regulated, adjusting a variable transmission element of the loaded line, to produce a transmission change correspondingto Ithe observed change lin resistance, and varying with frequency in 'a manner complementary to -thatof the loaded) line, on the' assumption that thel loading coils have undergone the Vsame temperature change as the other elements of the line, transmitting 8. In a-system for regulating transmission',

a loaded line in which iiiresponse toa given change in temperature therejsult'a'nt' transmissionchange varies With frequency, a pilot circuit having characteristics similar to those ofthe loaded line, means to indicate the change in resistance of the pilot circuit re'- in resistanceto produce a transmission change varying withfrequency in a manner complementary to that which would result in the "loaded'line in response to the change in temperaturelifall parts of tlie line including the loa'dingjcoilsr-had undergonethe same tem- ,peiatjure"change5-aneans to transmit- `a pilot' .Waveovera'circuit similar t o` said loaded lineand,I subjected' to similar transmission adjustments,fmeans to' indicate after such transmission adj ustinentl any .departure from the n ormal'level..ofsaid;pilot Wave due to a lag in the'temper'ature `changeof he loading coils ofthe circuit with" respect to other elements.

of lthe circi'iitqa, [secondadjustable transmisl .85.1 to adjust said second transmission element .to compensate for the indicatedc'change of level-A sion elem'ent in' Said loadedliiie, andfmeans of said pilot Wave.

t. In a system' for regulatingtransmission, a loaded line 1n Which in response to a. given change. in temperature the resultant transmission change varies With frequency, a pilot circuit having characteristics similar to those of the loaded line, means to indicate the change in resistance of the pilot circuit resulting from a change in temperature an adjustable transmission elementin said oade'd line, means to .adjust said transmission element in accordance with the indicated change in resistancevv to proluce a 4transmission, change varying With frequency in a manner complementary to that .Which Would result in theloaded line in response to the change in temj perature if all parts of the line including the loading coils had undergone the same' temperature change, means to transmit a pllot Wave over a circuit similar to said loaded l'ine and subjected to similan transmission adjustments, means to indicate any departure from the'i'iormal level ofsaid pilot Wave, a second adjustable transmission element in said load-v l ed line, and means to adjust said second trans- 'imi-v mission element to compensate for the indicated 'change' in level.

5. In-a system for regulating transmission, a loaded line in which in response to 'a given change in temperature the resultant transmission change varies Witlifrequency, a pilot circuithaving characteristics similar to those of the loaded line, means to indicate the change -in .resistance of the pilot circuit resulting from a change in temperaturean adjustabletransmission element in .said-loadi f .ed line, means'controlled by saidindicati'ng means for automatically adjusting sai-d transmission elemeiit in accordance With the 'indi- 'cated-'change in resistance to produce a trans result inthe loaded line in response'to the in temperature if all parts of the line chang inclu ng the loading coils-had undergone the saine temperatui'e change` means to transmit ia pilotn'i'ii'aowr a circuit similar to said loaded line and subjected to similartransmission adjustmentmeans to indicate after such transmission adjustment any departure from the normal level of said pilot wave due to a lag in 'the teinpe ature change of' the loading coils of the circuit with respect to otliei elements of the circuit, asccond adjustable transmission element in said loaded line, and means automatically controlled by said i last-mentioned indicating means to automatically adjust said second transmission element to compensate for the indicated change of level of said pilot wave.

6. Ina system for regulating-transmis sion, a loaded line in WliiclLinresponse to a given change i'n temperature the resultant transmission change varies with frequency, a pilot circuit having characteristics similar to those of the loaded line. means to indicate `the change in resistance of the pilot circuit resulting from a change in temperature, an adjustable transmission element iiilsaid loaded line, means controlled by said indicating means for automatically adj usi ing said transjlnission element in accordance With the indient-ed change in resistance to produce a transmission change varying with frequency ina inaniier con'ipleinentary to that Which Wouldijesult .in the loaded line in response to the change in temperature if' all parts of the line including the loading coils had undergone the sainetemperature change, means to transmit a pilot wavel over a circuit similar to said loaded line and subjected to similar transmission adjustment, means to indicate any departure from the normal level of said pilot wave, a second .adjustable transmission element in said loaded line, and means controlled by said last-inentioned indicating` means -to automatically adjust said second transmission element to compensate for the indicated change in level.

' 7. In a system for regulating transmission, a loaded line in Which in response to a given change in temperature the resultant transmission change varies with frequency, a pilot circuit having characteristics similar to those of the loaded line, means to indicate the change in resistance of the pilot circuit resulting from a change of temperature, an amplifiervin said loaded line and 'an adjustable potentiometer associated with said am# plier, said potentiometer including reactalice elements, means to adjust said potentiometer With its reactance elements in accordance With the indicated change in resistance to produce, by the cooperation of said potentiometer and amplifier, a transmission change varying with frequency in a manner complementary to that Which Would result in the loaded line in response to the change in teineratiire if all parts of thel line including the oading coils had undergone the same tcmperature change, means to transmita pilot Wave over a circuit similar to said loaded line and subjected `'to similai` transmission adjustments, meansf-toeindicate after such transmission adjustment any departure from the normal le vel of said pilot Wave diie to a lag iii thetemperature change of the loading coils of the circuit .with respect to the other 'elements of the circuit, an adjustable transms sion element associated with saidl loaded line .and capable of adjustmentto produce vary-` ing degrees of transmission change With frej qiiency,and means to adjust said transniission lsa element to com ensate for the indicated chan e of level o said pilot Wave. A l 8. n a system for regulating transmission,I a loaded line in which` in response toa' given change in temperature the resultant transf 4missionchaiige varies with frequency, a pilot vwith the indicated'changein resistance tov prod-nce, by the cooperationof said potenti' ometer and amplier, a trinsins'sion change varying with frequency in a manner complementary to that which would`V` result in the. M

loaded line in response to the change in teili'-, perature 'if' all parts of the line including the loading coils,had undergone thesametemperature change, means to transmit a pilot wave over a circuit similar to said loaded lline and subjected to similar transmission,

adj ustinentshmeans to indicate any departure from the normal level of saild pilot wave,

an adjustable transmission-element in Isaid loaded line capable of being adjusted to pro- 11 duce varying degrees of transmission change With frequency, vand means to adjust si `d. transmission 'element to4 compensate for t e indicated change ISin level. A

In testimony whereof, I have signed my name to this specification this 18th day of October 1928.

Y MANVEL K. ZINN; 

